1. Field of the Invention
This invention relates in general to a direct address storage device (DASD), and more particularly to a method and apparatus for reducing the servo position error signal non-linearity during self-servo writing irrespective of the head width.
2. Description of Related Art
Increased levels of storage capacity in floppy and hard disk drives are a direct result of the higher track densities possible with voice-coil and other types of servo positioners as well as the ability to read and write narrower tracks by using, for example, magneto resistive (MR) head technology. Previously, low track density disk drives were able to achieve satisfactory head positioning with leadscrew and stepper motor mechanisms. However, when track densities are so great that the mechanical error of a leadscrew-stepper motor combination is significant compared to track-to-track spacing, an embedded servo is needed so that the position of the head can be determined from the signals it reads.
Conventional hard disk manufacturing techniques including writing servo tracks on the media of a head disk assembly (HDA) with a specialized servo writer instrument. Laser positioning feedback is used in such instruments to read the actual physical position of a recording head used to write the servo tracks. Unfortunately, it is becoming more and more difficult for such servo writers to invade the internal environment of a HDA for servo-writing because the HDAs themselves are exceedingly small and depend on their covers and castings to be in place for proper operation. Some HDAs are the size and thickness of a plastic credit card. At such levels of microminiaturization, traditional servo-writing methods are inadequate.
Conventional servo-patterns typically comprise short bursts of a constant frequency signal, very precisely located offset from a data track's center line, on either side. The bursts are written in a sector header area, and can be used to find the center line of a track. Staying on center is required during both reading and writing. Since there can be between seventeen to sixty, or even more, sectors per track, that same number of servo data areas must be dispersed around a data track. These servo-data areas allow a head to follow a track center line around a disk, even when the track is out of round, as can occur with spindle wobble, disk slip and/or thermal expansion. As technology advances to provide smaller disk drives, and increased track densities, the placement of servo data must also be proportionately more accurate.
In magnetic disk drives, magnetic heads and recorded servo code in a track following servo mode are used for keeping the magnetic heads track centered during reading operations. The magnetic heads comprise a magnetic core having an air gap therein and having a coil wound thereon. These magnetic cores vary in effective magnetic widths due to their design and due to the manufacturing process. These physical variations among the magnetic heads result in variations in servo gain when they are individually connected in the servo loop.
The propagation width for the disk drive is selected according to the widest head to scale the erase bands near a constant. This implies, given a certain distribution in the components, a degradation in servo position error signal (PES) linearity due to very narrow heads.
It can be seen that there is a need for reducing the servo position error signal non-linearity during self-servo writing irrespective of the head width.